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Coverage-dependent adsorption studies of benzoic acid on a Cu(110) single crystal surface resulted in a sequence of novel, large periodicity structures containing differently oriented benzoate species, as shown by LEED, STM and FTIR. At low coverage, only flat-lying benzoate species are present and faceting of step edges is observed. The favorable mobility of flat-lying species results in formation of large, single domain islands with a [Formula: see text] periodicity containing four molecules per unit cell. Additional adsorption produces island growth, with single domains typically covering entire terraces and, simultaneously, addition of a fifth benzoate species per unit cell oriented perpendicular to the surface. This structure, denoted as the “α phase,” corresponds to a local coverage of θ=0.21 ML. Then, the periodicity changes to a [Formula: see text] structure with single and paired rows of upright species, retaining the dominant <4, 3> direction and large single domain sizes, in addition to six flat-lying species. The second structure, referred to as the “β phase,” corresponds to a local coverage of θ= 0.23 ML. The saturation coverage, c(8×2) structure, at θ=0.25 ML, grows in from step edges and contains only upright benzoate species. The results are interpreted in terms of the interplay between molecular orientation, site-specificity, diffusion and the intermolecular interactions favoring long range, two-dimensional order.

We have determined the azimuthal orientation of an adsorbate on a metal surface from an intramolecular-transition-derived feature in reflectance anisotropy spectroscopy (RAS). Adsorption of 9-anthracene carboxylic acid onto p͑2 3 1͒O͞Cu͑110͒ led to an ordered structure with a strong (2%), derivativelike feature at 4.5 eV. Fresnel theory predicts the measured intensity, functional behavior, and sense of the RAS signal for the molecule aligned along [110]. IR measurements confirm that the molecular plane is perpendicular to the surface and STM measurements support the azimuthal orientation. We reassign the sense of the clean Cu (110) surface RA spectrum. [S0031-9007(98)06135-3] PACS numbers: 68.45. 33.20.Kf, 73.20.At, Reflectance anisotropy spectroscopy (RAS), also known as reflectance difference spectroscopy (RDS) using linearly polarized, visible light to detect azimuthal dependence in surface structure, has evolved from fundamental semiconductor surface studies to the control of semiconductor processing in situ under higher pressures and with real-time feedback [1]. Early applications of the technique to metal surfaces showed strong anisotropy of fcc (110) surfaces associated with transitions between surface states as well as interband transitions in the near surface region [2,3]. Recently, there have been several attempts to follow the effects of molecular adsorption on metal surfaces [4][5][6][7]. Although the molecule-surface bond clearly modifies the inherent anisotropy of the substrate metal electronic states, the information is not directly interpretable and may contain little information regarding the orientation of nonbonding moities in multifunctional species. To date, none of the adsorbates studied [4-6,8] have shown an intramolecular electronic transition within the 1.5-5.5 eV energy range available. To demonstrate the potential of RAS, we chose a larger conjugated molecular species, so as to shift the intramolecular electronic transitions into the visible region. We report here the first observation of reflectance anisotropy originating from an intramolecular transition, following the adsorption of 9-anthracene carboxylic acid (9-AA) onto the p͑2 3 1͒O͞Cu͑110͒ surface, with a structure approximately described as p͑8 3 1͒g. No previous studies of this adsorption system exist.In principle, the azimuthal orientation of an adsorbate can be determined from the RA spectrum if a well-defined transition dipole, relative to the molecular coordinates, exists within the available energy range and the metal does not screen the E field of the light parallel to the metal surface. In favorable cases, such as the one chosen here, the application of RAS to molecular adsorbates requires understanding (i) the intensity; (ii) whether an adsorbate transition causes an increase or decrease in reflectivity, since the sense corresponds to a rotation of the adsorbate by 90 ± , and (iii) whether or not Fresnel equations, which are valid in the continuum limit, apply at microscopic dimensions and describe the substrate respon...